Though the Ecocomics post explains diminishing marginal utility well, I wanted to include something reader Chris emailed to me too, on account of how rad it is. Here I go, quoting from private correspondence without permission:

"The concept of declining marginal utility isn't limited to money, either; as the quantities of anything go up, each successive unit is probably worth less to the recipient than the last. If, for example, I met you on the street and said, "Hi, Ryan, let me buy you a hot dog," you might be delighted. If, when you were done, I said, "Let me buy you another one," you would probably be less so (depending on how hungry you are, and how much you like hot dogs). Eventually, if I kept buying you hot dogs, you would reach a point - no matter how much you like hot dogs - where the magic of free hot dogs would fade."

The magic of free hot dogs would fade. If there were ever a time for a frowney smiley, this is it: :(